CATALYTIC SITESFOR DEUTERIUM EXCHANGE WITH BENZENE OVER ALUMINA
4323
Catalytic Sites for Deuterium Exchange with Benzene over Alumina by P. C. Saunders’ and Joe W. Hightower* Department of Chemical Engineering, Rice University, Houston, Texas 77001 (Received February I d , 1970)
Some of the finer details of the interaction of benzene with the surface of an alumina catalyst have been investigated with the use of deuterium tracers. Exchange of benzene H atoms with DZoccurred readily at room temperature, but intermolecular redistribution of H and D atoms between CeHe and CeDe occurred at least 10-20 times as rapidly under the same conditions. Both reactions apparently took place on the same sites whose concentration was 6-15 x 1012 sites/cm2 as determined by COZpoisoning. Radioactive C1Q2 was used to confirm complete adsorption. Several other compounds (NO, CO, 02,HzO, and “3) had little poisoning effect on the exchange reaction. The exchange reaction with Dz exhibited a primary kinetic isotope effect by being faster than CeDe-Hz exchange; the kCH/kcD = 2 was attributed to a dissociative exchange mechanism in which CH (or CD) cleavage wa3 rate limiting. Benzene also poisoned the Hz-D2 equilibration. Speculation about the active sites led to the conclusion that all these exchange reactions perhaps occur on exposed Ala+ surface ions.
Introduction A characterization of the chemical nature of catalytically active sites on alumina has been the object of numerous papers spanning several years. Direct spectroscopic examination of surface functional groups, such as. lattice terminating OH groups or A13+ ions, and changes which they undergo during adsorption have led to identification of Lewis acid, Brgnsted, electron transfer, and perhaps other sites on alumina. Despite these advances, there remains considerable speculation about which, if any, of these sites are responsible for hydrocarbon catalysis. It is entirely possible that no single type of site is responsible for all reactions and that in some cases different sites must work in coordination to produce the desired catalysis. For this reason it is important to investigate a range of selected reaction types designed to probe the chemical nature and possibly to determine the concentration of active sites. One such test reaction is the exchange of hydrogen atoms with D2 under mild conditions. Larson and Hallza studied D2 exchange with methane, while Pink and coworkersZb chose exchange with another paraffin, propane, for their test reaction. Alumina is also an excellent catalyst for exchanging vinyl H atcms in olefins with Dz without ~ a t u r a t i o n ,and ~ ’ ~this technique has been used to prepare relatively pure perdeuterated 01efins.~ The density of active sites was determined for these reactions by selective poisoning techniques. l t 4 Using D2 exchange with aromatics as test reactions, McCosh and KemballG found ring H exchange rates over a (probably) X-alumina sample comparable with the methane rate,2a no ring directing effects due t o methyl substitution, no multiple exchange, and very slow side group exchange. The purpose of this report is to describe some of the finer details of the exchange of Dz with benzene7 carried out over the same y-, ?-alumina catalyst used in the methanel arid olefin3-5 reactions.
Experimental Section Catalyst. The 0.10-g catalyst sample was probably a mixture of y- and 7-alumiha prepared from the neutral hydrolysis of very pure aluminum isopropoxide by the MK Research and Development Co. of Pittsburgh, Pa.8 Its BET-N2 surface area was 158 m2/g, and metallic impurities (other than Al) were less than 50 ppm. “Standard pretreatment” included slowly raising the sample temperature under vacuum to 530”, treating it at temperature with 150 Torr 0 2 for 1 hr, and evacuating the catalyst for 2 hr to a “sticking” NIcLeod vacuum. It was then isolated from the vacuum system and cooled to reaction temperature. Both the catalyst and pretreatment are similar to those used in the previous s t ~ d i e s . ~ ~ f ~ - ~ The same catalyst sample was used in all these experiments, the “standard pretreatment’’ being performed before each experiment. Some irreversible poisoning occurred in the first few reactions, as the exchange activity decreased about 30% between the first and tenth runs starting with a fresh catalyst (see Table I). However, after that the catalyst appeared to be fairly well stabilized, and the initial exchange rates were reproducible to within f 5%.
* To whom all correspondence should be sent. (1) OCS Training Center, Fort Belvoir, Va.
(2) (a) J. G. Larson and W. K. Hall, J . Phys. Chem., 69, 3080 (1965); (b) B . D. Flockhart, S. S.Uppal, I . R. Leith, and R. C. Pink, Preprints of Papers from I V International Congress on Catalysis, Moscow,eompiled by J. W.Hightower, 1969, p 1429. (3) J. W. Hightower and W. K . Hall, J . Catal., 13, 161 (1969). (4) J. W. Hightower and W. K. Hall, Trans. Faradag Soc., 66, 477 (1970). (5) J. G. Larson, J. W.Hightower, and W. K. Hall, J. Org. Chem., 31, 1225 (196G). A , 1555 (1968). (6) R. McCosh and C. Kemball, J . Chem. SOC. (7) P. C. Saunders, M.S. Thesis, Rice University, Houston, Texas, 1969. (8) W. K. Hall and F. E. Lutinski, J . Catal., 2, 518 (1963).
The Journal of Physical Chemistry, Vol. 74, N o . 26, 1970
4324
P. C. SAUNDERS AND JOEW. HIGHTOWER
Table I: Rate Parameters from Isotopic Exchange and Redistribution Reactions over Alumina; Some Poisoning Effects Amount poison
Run no.
Temp,
kg' or
OC
M
kgP/kg or
tlhb
kuR/ku
mins
Catalyst aging 1 4 5 9
20 20 20 20
7.42 6.85 6.61 6.00
1.09 1.07 1.06 1.09 C~HE-DZ exchange and poisoning
11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 34 50 10 67
5.32 5.36 5.69 5.23 5.27 5.34 5.42 5.07 5.03 4.99 5.03 4.98 4.76 4.76 4.96 4.74 4.96 5.03 5.16 6.57 9.13 3.53 13.90
1.08 1.06 1.12 1.02 1.09 1.12 1.10 1.10 1.08 1.09 1.09 1.08 1.05 1.05 1.08 1.06 1.08 1.05 1.06 1.04 1.12 1.04 1.14
COZ
coz COZ coz coz COZ coz coz c02 COZ COZ 02
HzO
co co
NHs NHs NO NO
4.17 2.63 5.51 7.05 1.46 8.10 9.15 5.52 11.99 6.40 13.20 14.37 9.02 14.77 48.92 14.40 48.32 14.56 48.64
0.53 0.66 0.44 0.32 0.81 0.24 0.18 0.44 0.06 0.36 0.03 0.99 0.80 0.79 0.73 0.88 0.62 0.90 0.86
57.2 56.8 53.5 58.2 57.8 57.0 56.2 60.0 60.5 61.0 60.5 61.4 64.2 63.9 61.4 64.2 61.4 60.5 59.0 46.5 33.5 86.6 22.0
C ~ D E - Hexchange ~ 34 35
20 20
2.73 2.75
0.99 1.03
117.4 116.5
C~HE-C~DE redistribution, poisoning 36 37 38 39
20 20 20 20
coz
48.7 41.3 46.9 44.8
c02
coz
coz
10.73 13.20 5.61
a k+ and k, represent the number of H (or D ) atoms entering 100 molecules per minute. one-half the equilibrium value.
+
Reactants. The benzene was 99 % Fisher B-414 grade which was dried over5 8 molecular sieves,vacuum distilled, and carefully outgassed before use. Perdeuterated benzene (Merck Sharp and Dohme, Ltd.) was similarly treated and had an isotopic purity of 99.5% D. D, gas, which was prepared from Chemical Samples Co. 99.7% DzO in a Elhygen electrolytic ultrapure deuterium generator, was %owed through a trap thermostated at - 195'. Research grade hydrogen was similarly passed through a -195' trap. Helium was purified by diffusion through a qua_rtz thimble manufactured by Electron Technology, Inc. The gases used as poisons (COZ,NO, HzO, "3, and The Journal of Physical Chemistry, Val. 74, No. 86,1970
b
0.13 0.07 0.41
2.9 3.5 3.1 3.2
t i / , is the time required for d, or u to reach
0 2 ) were outgassed and distilled if condensable or passed through thermostats at - 195 or -78". Radioactive C1402 was prepared by HNOs acidification of Mallinckrodt Nuclear barium carbonate-C-14; concentrated starting material was purified by glpc, outgassed, distilled, and diluted to an appropriate activity level before use. Equipment. All reactions were carried out in an allPyrex recirculation reactor shown in Figure 1. Approximately 85% of the total 354-cc reactor system volume was in the spherical mixing bulb b. I n addition to a vertical reactor a, single-action magnetically driven Pyrex pump c (about 300-cc/min capacity), and
4325
CATALYTIC SITESFOR DEUTERIUM EXCHANGE WITH BENZENE OVER ALUMINA
Figure 1. Recirculation system: a, reactor; b, mixing chamber; e, glass pumps; d, poison doser; e, capillary leak t o mass spectrometer.
mixing volume, the recirculation loop featured a 1.97cc bypass doser d located immediately upstream from the catalyst bed. A 2-ft long Kemball-typeg Pyrex capillary leak e connected the recirculation loop directly to the ion source of the mass spectrometer. This leak was sufficiently small that only 0.3%/hr of a reaction mixture was lost under normal operating conditions (1 10 Torr in reactor system). Deuterium analyses were made on an ion-pumped CEC 21-104 mass spectrometer under low-voltage conditions where -H fragmentation was negligibly small (much less than 0.1%). Realtive sensitivities for C6He and CeDBwere measured with a micromanometer and accounted for in the calculations, and the usual Cla isotopic impurity corrections were made. Radioactivity analyses were carried out in a small metal-valve-isolated chamber containing a mercury manometer and separated by a 0.001-in. thick mica windowfr om agas flow open end Geiger-Muller counter. A Nuclear-Chicago Model 192A Ultrascaler accumulated the pulses for a preset time. Procedures. After the catalyst had been pretreated, bypassed, and cooled to reaction temperature, a standard benzene-D2 (10 : 100 Torr) mixture was thoroughly mixed in the recirculation loop. At time zero the mixture was admitted to the catalyst by manipulation of the two reactor bypass stopcocks, and scans over the mass range from m/e 78 to 85 were taken at regular intervals on the mass spectrometer. In the redistribution experiments, mixtures of 5 :5 : 100 = CaH&.&DaHe were used, while the HgDz equilibrations involved equal amounts of the two reactants whose total pressure was 100 Torr. “Back exchange” reactions made use of C&--H2 (10: 100 Torr) mixtures. I n the poisoning experiments, a known pressure of the gaseous test compound was first carefully measured into the calibrated doser, and after the reaction rate had become well established (24 min in the exchange experiments and 8 min in the redistribution reactions), the recirculation stream was diverted through the
doser, and the poison was carried as a slug directly to the catalyst. Radioactive COzpoison was used as a probe to ascertain whether all the poison was adsorbed on the catalyst. The appearance of ,radioactivity in the gas phase after C1402injection would indicate incomplete adsorption, and specific activity (counts min-I mm-’ in counter) measurements of aliquots of the gas phase would determine the fraction of that material adsorbed. Treatment of Data, The linear Kemball equations1° were used to describe the course of the exchange reactions, 4, of benzene with Dz, and the first-order rate constant k , is a measure of the number of D atoms entering 100 hydrocarbon molecules per minute. 4represents the statistical equilibrium value of 4 based on the initial benzene-Dz ratio (isotope effects” were neglected in this treatment). The rate constant for the disappearance of the lightweight material is Ice, and the ratio k,/ko measures the multiplicity, 34, of the exchange reaction. Rate equations for the redistribution reactions are similar to those derived by Bolder, et aZ.lZ An extent of reaction variable, u, was defined as 3
u
=
6
Cid, + iC(6 - i)di i=O =4
where i is the number of D atoms per molecule, and d, is the fraction of total hydrocarbon atoms which contain i D atoms. The rate constant for the redistribution reaction, k,, may be determined from the slope of plots of In (u, - u) us. time where u, is the equilibrium (assumed to be a binary distribution) value of u. The Hz-D2 rate constant, k H D , was calculated from the slope of In (HD, - HD) vs. time plots.
Results A summary of most of the experimental results is contained in Table I. c6Hg-D~ Exchange. After the initial catalyst deactivation (runs 4-10], the activit,y for C6&-Dz exchange remained essentially constant ( k , = 5.1 i 0.2 min-’) with a half-time of about 1 hr. In all cases the multiplicity, M , of the reaction was near unity. COZ was an effective poison for this reaction. Whereas the unpoisoned “4 plot” remained linear until equilibrium was approached (Figure 2, curve A), addition of COZ via the dosing device caused an abrupt break in the curve (Figure 2, curve b) which was followed by a new straight line from which a poisoned rate constant, k , p , could be calculated. Figure 3 (curve a) shows the ratio of the rate constants for several runs in which various amounts of COz were added. The titra(9) C. Kemball, Proc. Roy. SOC., Ser. A , 207, 539 (1951). (10) C. Kemball, Advan. Catal. Relat. Szibj., 11, 223 (1957). (11) E. F. Meyer and C. Kemball, J. Catal., 4, 711 (1965). (12) H. Bolder, G. Dallinga, and H. Kloosterziel, ibid., 3, 312 (1964).
The J O U T ~ ofUPhysical ~ Chemistry, Vol. 74, N o . 86,1970
4326
t
P. C. SAUNDERS AND JOEW. HIGHTOWER
zol 0 ' I
0
aeI
0
10
20
30
40
0
.
I
. . 30
.
,
,
,
.
,
60 90 Time (mins.)
,
'
120
,
'
'
150
' ' .
J
ieo
.'
Figure 4. Test for COZadsorption by radioactivity measurements; effect on CeHs-Dz exchange rate.
Time (mins.)
Figure 2. Rate curves for CaHgDz exchange; effect of COZ poisoning.
e,
-- -
d- N H ~ e - co
sb Figure 3. Exchange activity reduction by selected poisons.
tion curve intercept occurs at about 15 X 10'2 COz molecules/cm2. None of the other compounds tested was nearly so effective as COZin poisoning the reaction, as indicated by curves b, c, d, e, and f in Figure 3. It is doubtful that a significant fraction of the NO, CO, or O2 was adsorbed, since mass spectral peaks, although small, were observed for these compounds, and the peak heights were proportional to the poison dose size. An Arrhenius plot of the rate constants between 10 and 67" (runs 30-33) was linear with an activation energy of 4.3 f 0.3 kcal/mol. C&-H2 Exchange. The reverse exchange reaction, C6D6-HZ (runs 34-35), occurred only about half as fast (IC, = 2.74 f 0.01, tl/, = about 2 hr) as did the forward reaction previously described. Again the 4 plots were linear. To test for complete adsorption of the C02, radioactive CI4O2was used as the poison and the gas phase The Journal of Physical Chemistry, Vol. 74,No. 26, 1970
was monitored for radioactivity; the results are shown in Figure 4. At 24 rnin the "lethal dose" of Cl4O2, e.g., just enough C1402necessary to poison the catalyst completely (note break in 4 plot), was added via the doser. The first radioactivity measurement at 25 min showed some activity, but after 38 min there was no measurable activity in the gas phase; hence, all the C02 was adsorbed. At 42 rnin the catalyst was bypassed, and a second identical Cl4OZpoison dose was added. Two measurements (at 47 and 54 min) in the bypassed condition indicated the activity level expected from the first C1402dose had there been no adsorption. All activity measurements in Figure 4 are normalized to this value. At 58 rnin the catalyst was again opened to the recirculation system, and some additional adsorption of Ci402 (about 30% of the second dose) occurred. Beginning at 83 min, the catalyst temperature was increased rapidly in three separate steps and maintained at the indicated temperature between each jump. At 96" all the Cl4OZadsorbed in the second dose and about 12% of the first dose was desorbed. The exchange activity remained poisoned, however, until 50-60% of the adsorbed CI4O2from the first pulse was desorbed at 222". All the C1402was essentially desorbed at 400", for the specific activity of the gas phase was about double that for a single dose. C&,3-C,3& Redistribution. The redistribution of H and D atoms between benzene molecules occurred much more rapidly than did either the c6He-D~ or CeDe-Hz exchange reactions. The rate constants were about an order of magnitude greater at 20", and the times required for half-conversion to equilibrium were ~ / Z O as long as that for the C6H6 Dz exchange reaction. C02 also poisoned this reaction, and the amount necessary for complete catalyst deactivation (15 X 1OlZ CO2 molecules/cm2) was the same as that for the exchange reactions (see solid points, curve a, Figure 3). H2-D2 Equilibration. The rate for H2-Dz equilibration was extremely fast as indicated by curve a in Figure
+
CATALYTIC SITESFOR DEUTERIUM EXCHANGE WITH BENZENE OVER ALUMINA 01
1
2
‘.
0
5 Time
10
15
(mins.)
Figure 5 . Effect of CsHs on HQ-D~ equilibration rate.
5 . However, when approximately 1 Torr benzene was added to the starting H2-Dz mixture, the equilibration reaction was retarded by at least three orders of magnitude (see curve b, Figure 5 ) . Effect of Deuterated Catalyst. Exchanging the 2 X 1014H atoms/cm2 intrinsically on the catalyst had no apparent affect on any of the results. The exchange and redistribution rates were identical on both hydrogenated and deuterated catalysts. No exchange between H atoms in the benzene and surface H atoms was observed.
Discussion These results, coupled with other exchange res u l t ~ over ~ &an~ alumina ~ ~ ~ catalyst from the same GA48 batch which had been treated in an equivalent manner, have clearly demonstrated the ability of alumina to activate CH bonds in hydrocarbons under mild conditions. Furthermore, the catalyst exhibits considerable discrimination between different types of bonds, the order of exchange rates being aromatic ring C H > olefinic vinyl CH3v4 > paraffinic CH.2a The GA-48 used in these experiments is apparently about two orders of magnitude more active than the catalyst used by McCosh and Kemball,6 and the activation energy (4.3 kcal/mol) is significantly lower than the value they obtained (6.0 kcal/mol) for the same reaction. Although their absolute activities varied markedly with outgassing temperature, Flockhart, et ~ 1 .found ) ~ ~a still higher activation energy (8.7 kcal/ mol) for propane exchange with D2over a cataIyst similar to that used by McCosh and Kemball. These observations illustrate some of the dangers in comparing absolute rate data taken from different catalysts and casts doubt on some of the conclusions drawn by McCosh and Kemball. It appears that rather than hav-
4327
ing the same activation energies, each bond type may have its own unique activation energy with that for aromatic ring being lower than those for olefinic or for paraffinic C-H bonds. While the absolute values may vary from one catalyst to another, the ratio of the activation energies apparently remains about the same in each system. For example, for GA-48 the aromatic ring-paraffinic (methane) ratio is 4.3:5.7 kcal = 0.8, and for the McCosh-Flockhart type catalyst the ratio is 6,0:8.7 kcal = 0.7 for aromatic ring-paraffinic (propane) exchange. Also, the relative exchangeability of the ring H atoms and side group H atoms McCosh and Kemball observed in the alkyl benzenes compares favorably with the relative exchangeabilities of benzene and methane atoms over GA-48 under similar conditions. All investigators2-416 have observed that the reaction multiplicity, M , is unity and have concluded that the exchange reactions are all stepwise. While this is likely the case, there is another factor which could account for M being one and which could obscure the true nature of the exchange reaction. This factor is the extremely rapid intermolecular redistribution reaction which occurs at a rate 10-20 times as fast as the exchange with Dz. A necessary and sufficient condition for M = 1 requires only the maintenance of a statistical equilibrium distribution among all exchangeable positions at all times within the hydrocarbon; this condition was assured by the rapid redistribution reaction regardless of the multiplicity of the initial but slower D2 exchange reaction. Moreover, this scrambling reaction can also explain the absence of ortho-paradirecting effects which McCosh and KembalP did not observe in the ring exchange of alkylbenzenes. The rapid redistribution occurs only among olefinic vinyl4 and aromatic ring CH bonds; paraffinic CH bonds are not so involved. The poisoning experiments have supplied considerable information about the concentration and chemical nature of the active sites, Perhaps the most surprising result was the absence of significant poisoning with NO, CO, HzO, 0 2 , and NHs. It is possible that benzene is so strongly adsorbed as to prevent adsorption of these compounds; mass spectral analyses indicated that NO, CO, and O2 were not adsorbed to an appreciable degree; adsorption of H2O and NHa were not tested. The fact that NO is not a poison does not necessarily rule out the A13+Lewis acid NO adsorption site, identified by Lunsford13 using an epr technique, as being operative in these reactions. The site concentration by NO spin density measurements was 4.9 X cm2,and Lunsford suggested that they may be the same as the 5.2 X 1012/cm2a sites identified by Peril4 using l-butene to titrate the adsorbed C02 ir spectrum. An(13) J. H.Lunsford, J . Catal., 14, 379 (1969). (14) J. B.Peri, J . Phys. Chenz., 70, 3168 (1966). The Journal of Physical Chemistry, VoL. 74, N o . 96,1070
4328 other explanation is that the temperature (20") was not high enough for NO to overcome the activation energy barrier for adsorption on the active sites; Larson and Hallza found NO did poison methane exchange when added at a higher temperature. CO is known not to adsorb on alumina to a significant degree; its ir spectra is discussed at length by Parkyns.16 The inert behavior of CO is thus not unexpected. Water caused some poisoning, but its effect was small compared with the devastating effect of COz. This is likely due to nonselective adsorption on activated alumina, as suggested by Larson and At room temperature, 0 2 is reduced only very slowly by hydrogen (or Dz) to form water on alumina. Larson and Hall found this reaction also poisoned their methane exchange when the temperature was increased sufficiently to activate the reaction. They postulated the formation of water directly on the active sites in this way. The temperature was apparently too low for this reaction to occur in the present study. The fact that fairly large doses of ammonia caused no drastic poisoning suggests that the exchange reactions do not occur on the acid-base pair sites originally proposed by Peril4 Unfortunately, it is not known whether the ammonia,adsorbed at all nor if the NH bonds underwent exchange with Dz. An affirmative answer to this last uncertainty would clearly indicate that a chemical interaction had occurred between ammonia and the alumina surface, but this interaction (if it did occur) did not affect the exchange rate. The observation that there was no significant change in any of the exchange reaction characteristics when the catalyst was fully deuterated indicates that the vast majority of the catalyst H atoms, whose concentration is about 2 X 1014/cm2,scannot be involved in the reaction mechanism. Carbon dioxide was clearly the most effective poison tested. If one assumes a 1:1 = site-poison molecule ratio, the poisoning intercept in Figure 3 gives a site density of about 15 X loL2sites/cm2 for both the DZ exchange and the redistribution reactions. This value is the same as that observed for cyclopentene exchange with Dz4and Hz-Dz equilibration,16 even though the method of COz addition was different in the latter paper. However, this value must be taken as a maximum measure of the site concentration, as the COZmay also have been adsorbed nonselectively on sites other than those which catalyze the reactions. Supporting this contension is the nonlinearity of all the poisoning curves, and extrapolation of the initial slopes of the curves gives an intercept in the range 6-8 X 10l2sites/ em2 in all cases. (Another interpretation for the nonlinearity of the poisoning curves may be that the sites are inhomogeneous and that the most highly active sites are the first ones selectively poisoned.) Further evidence comes from the radioactive poisoning experiments. Whereas the catalyst could adsorb at least The Journal of Ph&eal Chemistry, Vol. '74, No. 26, 1970
P. C. SAUNDERS AND JOEW. HIGHTOWER 30% more COz than was necessary to poison the exchange reaction, the exchange activity did not recommence until the temperature was increased (between 96 and 222" in Figure 4) to the point where 12-60% of the first C1402dose was desorbed. This would indicate that a coverage in the range 6-13 X l0lz molecules/cm2 was sufficient to poison the reaction if they were all adsorbed selectively on the active sites. On the basis of these findings, we believe the true site density is probably about 8 X 1012/cm2. Peril4has shown by ir measurements that COzis held in a variety of ways on alumina, and most of the bands he observed have also been observed by Little and Amberg," Parkyns,lB and Gregg and Ramsay.lQ In his most thorough treatment of band identification, Parkyns18 divided the ir absorption bands observed when COz was adsorbed on alumina into two groups: those which were pressure sensitive and those which were time dependent. Neither of these categories can adequately explain the poisoning results, for the COz adsorption occurred fairly rapidly (not time dependent) and the adsorbed material on the active sites could not be removed except by heating to temperatures greater than 100' (not pressure dependent). The structures associated with these bands were probably physically adsorbed COZ,bicarbonate ions, or carbonate ions. On the other hand, a single band at 1780 em-' was neither pressure nor time dependent, nor was it removed by evacuation at 100°.lB This band is probably due to a linear structure of the form
+o-c=o
O-CtO
/
A1+
or
//
A1
involving especially exposed aluminum cations. 143 l8 This band was observed only on the y-alumina samples and was absent on the X-alumina prepared from aluminum trihydrate (Table I, ref 19). Since this latter was the same type catalyst used by McCosh and Kemball, perhaps the absence of this type GO2 adsorption site could account for their much lower activity level and higher activation energy than observed for the GA-48 catalyst for benzene exchange. These findings lead us to suggest that the Hz-D~equilibration, Dz exchange, and intermolecular HD redistribution reactions all occur on the same exposed A13+surface ions whose concentration is the order of 8 X 101%/cm2. The sites may be the same ones on which NO is weakly adsorbed13 and with which COZinteracts to give an ir band at 1780 cm-l.18 The D2 exchange re(15) N. D. Parkyns, J . Chem. SOC.A, 1910 (1967). (16) F. H. Van Cauwelaert and W. K. Hall, Trans. Faraday S O C . , . ~ ~ , 454 (1970). (17) L. H. Little and C. H. Amberg, Can. J . Chem., 40, 1997 (1962). (18) N. D. Parkyns, J . Chem. SOC.A, 410 (1969). (19) S. J. Gregg and J. D. F. Ramsay, J . Phgs. Chem., 73, 1243 (1969).
ADSORPTION OF CHaBr ON SILICA GEL actions are likely stepwise involving a dissociative mechanism whose slow step is CH cleavage. The much greater activity of vinyl or aromatic CH bonds than paraffinic CH indicates that the initial interaction with the surface may involve attraction of the ?r system in the molecule. Perhaps two molecules ?r bonded to the same or adjacent A13+ions could undergo the redistribution reaction. Strong adsorption of the ?r system in benzene probably inhibits adsorption of hydrogen on these same sites and accounts for the poisoning of the H2-D2 equilibration by benzene. Such H2-D2 poisoning is probably fairly common in oxide catalysis, having been recently reported by Conner and KokesZ0in the ethylenezinc oxide system.
4329 Now that these exchange sites have been somewhat better characterized, it is doubtful that they are the same ones which catalyze isomerization in olefins. Results from some experiments currently underway will hopefully resolve this uncertainty.
Acknowledgments. The authors are grateful to the Mobil Foundation, Inc., for a grant-in-aid which supported this work. P. C. s. wishes to acknowledge an NDEA student scholarship. Thanks are also due to Dr. Hall at Mellon Institute for kindly providing a sample of the GA-48 catalyst. (20) W. C. Conner and R. J. Kokes, J . Phys. Chem., 73, 2436 (1969).
Adsorption of Methyl Bromide on the Surface Hydroxyls of Silica Gel by F. H. Van Cauwelaert,* J. B. Van Assche, and J. B. Uytterhoeven University of Leuven, Laboratorium voor Oppervlaktescheilcunde, de Croylaan 42, SOSO-Heverlee, Belgium (Received January 26, 1970)
A study of the heat of adsorption of CHsBr on silica gel Aerosil reveals that the silica surface is heterogeneous with regard to the distribution of the hydroxyls. This distribution depends on the drying temperature. At low pretreatment temperatures the density in hydroxyl groups is such that more than one OH reacts with each CH3Br molecule. Nevertheless, in the thermodynamic data there is no support for the idea that a high amount of hydroxyls is located in pairs. A discussion of the adsorption mechanism of CHzBr and related compounds was made, based on literature data of the OH frequency shift AV and the ionization potential. It is shown that charge transfer contributes to the adsorption mechanism. However, to explain AV and subsequently the adsorption energy, other factors, especially electrostatic phenomena, must be taken into account.
IntroductionThe physical adsorption of gases on silica gel, and its relation to the structure of the silica gel surface, was investigated in numerous studies.’-+ I n these studies the surface hydroxyls are generally considered to be the most important adsorption sites. These hydroxyl groups are usually covered with adsorbed water which can be removed by drying at elevated temperatures eventually combined with a vacuum treatment. The hydroxyl density at the surface was discussed in several papers. Iler6 concluded that a fully hydrated surface holds about 8 hydroxyls/100 De Boer and Vleeskens? have shown that an annealing of the silica gel at 450” and a subsequent rehydrati2n resulted in a surface covered with 4.6 hydroxyls/100 A.2 This figure was supported by Fripiat and Uytterhoevem8 The picture developed by De Boer and Vleeskens? and by Fripiat and Uytterhoevens was not further supported in more recent work. Perig and Peri and Hens-
leyl0 tried to make a more detailed picture of the hydroxyl distribution on the surface of silica gel. These authors claimed that the silica gel surface cannot be described in terms of a random distribution of isolated hy-
* To whom correspondence should be addressed. (1) R. S. McDonald, J . Amer. Chem. Soc., 79, 850 (1957). (2) M. R. Basila, J . Chem. Phys., 35, 1151 (1961). (3) G. A. Galkin, A. V. Kiselev, and V. I. Lygin, Russ. J . Phys. Chem., 36, 951 (1962). (4) V. Ya. Davydov, A. V. Kiselev, and V. I. Lygin, Proc. Acad. Sci., USSR Phys. Chem. Sect., 147, 769 (1962). (5) A . V. Kiselev, Ya. Kontetski, and I. Chizhek, Proc. Acad. Sci., U S S R Phys. Chem. Sect., 137,283 (1961). (6) R. K. Iler, “The Colloid Chemistry of Silica and Silicates,” Cornel1 University Press, Ithaca, N. Y . , 1955, pp 242-247. (7) J . H . De Boer and J. M. Vleeskens, Proc. Kon. Ned. Akad. Wetensch., B61, 2 (1958). (8) J. J. Fripiat and J. B. Uytterhoeven, J . Phys. Chem., 66, 800 (1962). (9) J. B. Peri, ibid., 70,2937 (1966). (10) J. B. Peri and A. L. Hensley, &id., 72, 2926 (1968).
The Journal of Physical Chemistry, Vol. 74, No. 26, 1970
